Algol

Algol
	Location of β Persei (circled)
Observation data; Epoch J2000 Equinox J2000
Constellation	Perseus
Right ascension	03h 08m 10.13245s
Declination	+40° 57′ 20.3280″
Apparent magnitude (V)	2.12 (- 3.39)
Characteristics
Spectral type	Aa1: B8V; Aa2: K0IV; Ab: F1V (kA4hA9.5mF0:)
U−B colour index	−0.37
B−V colour index	−0.05
Variable type	EA/SD
Astrometry
Radial velocity (Rv)	3.7 km/s
Proper motion (μ)	RA: 2.99 mas/yr ; Dec.: −1.66 mas/yr
Parallax (π)	36.27 ± 1.40 mas
Distance	90 ± 3 ly ; (28 ± 1 pc)
β Per Aa1
Absolute magnitude (MV)	−0.07
β Per Aa2
Absolute magnitude (MV)	2.9
β Per Ab
Absolute magnitude (MV)	2.3
Orbit
Primary	β Per Aa1
Companion	β Per Aa2
Period (P)	2.867328 days
Semi-major axis (a)	0.00215″
Eccentricity (e)	0
Inclination (i)	98.70°
Longitude of the node (Ω)	43.43°
Orbit
Primary	β Per A
Companion	β Per B
Period (P)	680.168 days
Semi-major axis (a)	0.09343″
Eccentricity (e)	0.227
Inclination (i)	83.66°
Longitude of the node (Ω)	132.66°
Periastron epoch (T)	2446927.22
Argument of periastron (ω); (primary)	310.02°
Details
β Per Aa1
Mass	3.17 ± 0.21 M☉
Radius	2.73 ± 0.20 R☉
Luminosity	182 L☉
Surface gravity (log g)	4.0 cgs
Temperature	13,000 K
Rotational velocity (v sin i)	49 km/s
Age	570 Myr
β Per Aa2
Mass	0.70±0.08 M☉
Radius	3.48±0.28 R☉
Luminosity	6.92 L☉
Surface gravity (log g)	3.5 cgs
Temperature	4,500 K
β Per Ab
Mass	1.76±0.15 M☉
Radius	1.73±0.33 R☉
Luminosity	10.0 L☉
Surface gravity (log g)	4.5 cgs
Temperature	7,500 K
Other designations
	Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei, BD+40°673, FK5 111, GC 3733, HD 19356, HIP 14576, HR 936, PPM 45864, SAO 38592.
Database references
SIMBAD	data

Algol /ˈælɡɒl/,^[11] designated Beta Persei (β Persei, abbreviated Beta Per, β Per), known colloquially as the Demon Star, is a bright multiple star in the constellation of Perseus and one of the first non-nova variable stars to be discovered.

Algol is a three-star system, consisting of Beta Persei Aa1, Aa2, and Ab – in which the hot luminous primary β Persei Aa1 and the larger, but cooler and fainter, β Persei Aa2 regularly pass in front of each other, causing eclipses. Thus Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days during the roughly 10-hour-long partial eclipses. The secondary eclipse when the brighter primary star occults the fainter secondary is very shallow and can only be detected photoelectrically.^[12]

Algol gives its name to its class of eclipsing variable, known as Algol variables.

^ ^a ^b ^c ^d ^e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
^ ^a ^b ^c Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
^ ^a ^b Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.
^ ^a ^b Cite error: The named reference lestrade1993 was invoked but never defined (see the help page).
^ Cite error: The named reference frank2022 was invoked but never defined (see the help page).
^ Fletcher, Emery S. (1964). "Spectrophotometry of Algol". The Astronomical Journal. 69: 357. Bibcode:1964AJ.....69..357F. doi:10.1086/109284.
^ ^a ^b ^c ^d ^e ^f ^g Soderhjelm, S. (1980). "Geometry and dynamics of the Algol system". Astronomy and Astrophysics. 89 (1–2): 100. Bibcode:1980A&A....89..100S.
^ ^a ^b ^c ^d ^e ^f ^g ^h Baron, F.; Monnier, J. D.; Pedretti, E.; Zhao, M.; Schaefer, G.; Parks, R.; Che, X.; Thureau, N.; Ten Brummelaar, T. A.; McAlister, H. A.; Ridgway, S. T.; Farrington, C.; Sturmann, J.; Sturmann, L.; Turner, N. (2012). "Imaging the Algol Triple System in the H Band with the CHARA Interferometer". The Astrophysical Journal. 752 (1): 20. arXiv:1205.0754. Bibcode:2012ApJ...752...20B. doi:10.1088/0004-637X/752/1/20. S2CID 11110989.
^ ^a ^b ^c ^d ^e ^f Zavala, R. T.; Hummel, C. A.; Boboltz, D. A.; Ojha, R.; Shaffer, D. B.; Tycner, C.; Richards, M. T.; Hutter, D. J. (2010). "The Algol Triple System Spatially Resolved at Optical Wavelengths". The Astrophysical Journal Letters. 715 (1): L44–L48. arXiv:1005.0626. Bibcode:2010ApJ...715L..44Z. doi:10.1088/2041-8205/715/1/L44. S2CID 118573026.
^ Tomkin, J.; Huisong, T. (1985). "The rotation of the primary of Algol". Publications of the Astronomical Society of the Pacific. 97: 51. Bibcode:1985PASP...97...51T. doi:10.1086/131493. S2CID 119392903.
^ "Algol". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
^ "Beta Persei (Algol)". AAVSO. January 1999. Archived from the original on 8 July 2006. Retrieved 31 July 2006.

[hipparcos-1] Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.

[ducati-2] Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.

[gcvs-3] Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S.

[lestrade1993-4] Cite error: The named reference lestrade1993 was invoked but never defined (see the help page).

[frank2022-5] Cite error: The named reference frank2022 was invoked but never defined (see the help page).

[fletcher2964-6] Fletcher, Emery S. (1964). "Spectrophotometry of Algol". The Astronomical Journal. 69: 357. Bibcode:1964AJ.....69..357F. doi:10.1086/109284.

[soderhjelm-7] ^ ^a ^b ^c ^d ^e ^f ^g Soderhjelm, S. (1980). "Geometry and dynamics of the Algol system". Astronomy and Astrophysics. 89 (1–2): 100. Bibcode:1980A&A....89..100S.

[baron-8] ^ ^a ^b ^c ^d ^e ^f ^g ^h Baron, F.; Monnier, J. D.; Pedretti, E.; Zhao, M.; Schaefer, G.; Parks, R.; Che, X.; Thureau, N.; Ten Brummelaar, T. A.; McAlister, H. A.; Ridgway, S. T.; Farrington, C.; Sturmann, J.; Sturmann, L.; Turner, N. (2012). "Imaging the Algol Triple System in the H Band with the CHARA Interferometer". The Astrophysical Journal. 752 (1): 20. arXiv:1205.0754. Bibcode:2012ApJ...752...20B. doi:10.1088/0004-637X/752/1/20. S2CID 11110989.

[zavala-9] ^ ^a ^b ^c ^d ^e ^f Zavala, R. T.; Hummel, C. A.; Boboltz, D. A.; Ojha, R.; Shaffer, D. B.; Tycner, C.; Richards, M. T.; Hutter, D. J. (2010). "The Algol Triple System Spatially Resolved at Optical Wavelengths". The Astrophysical Journal Letters. 715 (1): L44–L48. arXiv:1005.0626. Bibcode:2010ApJ...715L..44Z. doi:10.1088/2041-8205/715/1/L44. S2CID 118573026.

[tomkin-10] Tomkin, J.; Huisong, T. (1985). "The rotation of the primary of Algol". Publications of the Astronomical Society of the Pacific. 97: 51. Bibcode:1985PASP...97...51T. doi:10.1086/131493. S2CID 119392903.

[11] "Algol". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)

[12] "Beta Persei (Algol)". AAVSO. January 1999. Archived from the original on 8 July 2006. Retrieved 31 July 2006.

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Observation data Epoch J2000 Equinox J2000
Location of β Persei (circled)
Constellation	Perseus
Right ascension	03^h 08^m 10.13245^s^[1]
Declination	+40° 57′ 20.3280″^[1]
Apparent magnitude (V)	2.12^[2] (- 3.39^[3])
Characteristics
Spectral type	Aa1: B8V^[4] Aa2: K0IV^[4] Ab: F1V^[5] (kA4hA9.5mF0:^[6])
U−B colour index	−0.37^[2]
B−V colour index	−0.05^[2]
Variable type	EA/SD^[3]
Astrometry

Radial velocity (R_v)	3.7 km/s
Proper motion (μ)	RA: 2.99^[1] mas/yr Dec.: −1.66^[1] mas/yr
Parallax (π)	36.27 ± 1.40 mas^[1]
Distance	90 ± 3 ly (28 ± 1 pc)


β Per Aa1
Absolute magnitude (M_V)	−0.07^[7]


β Per Aa2
Absolute magnitude (M_V)	2.9^[7]


β Per Ab
Absolute magnitude (M_V)	2.3^[7]

Orbit^[8]
Primary	β Per Aa1
Companion	β Per Aa2
Period (P)	2.867328 days
Semi-major axis (a)	0.00215″
Eccentricity (e)	0
Inclination (i)	98.70°
Longitude of the node (Ω)	43.43°
Orbit^[8]
Primary	β Per A
Companion	β Per B
Period (P)	680.168 days
Semi-major axis (a)	0.09343″
Eccentricity (e)	0.227
Inclination (i)	83.66°
Longitude of the node (Ω)	132.66°
Periastron epoch (T)	2446927.22
Argument of periastron (ω) (primary)	310.02°
Details
β Per Aa1
Mass	3.17 ± 0.21^[8] `M`_☉
Radius	2.73 ± 0.20^[8] `R`_☉
Luminosity	182^[7] `L`_☉
Surface gravity (log g)	4.0^[9] cgs
Temperature	13,000^[9] K
Rotational velocity (v sin i)	49^[10] km/s
Age	570^[7] Myr


β Per Aa2
Mass	0.70±0.08^[8] `M`_☉
Radius	3.48±0.28^[8] `R`_☉
Luminosity	6.92^[7] `L`_☉
Surface gravity (log g)	3.5^[9] cgs
Temperature	4,500^[9] K


β Per Ab
Mass	1.76±0.15^[8] `M`_☉
Radius	1.73±0.33^[8] `R`_☉
Luminosity	10.0^[7] `L`_☉
Surface gravity (log g)	4.5^[9] cgs
Temperature	7,500^[9] K

Other designations
Algol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul, β Persei, β Per, 26 Persei, BD+40°673, FK5 111, GC 3733, HD 19356, HIP 14576, HR 936, PPM 45864, SAO 38592.
Database references
SIMBAD	data